1. Field of the Invention
The present invention relates to an electrophotographic type image forming apparatus, more specifically relates to an image forming apparatus having transfer means contacting to an image bearing member through a low pressure, for electrostatically transferring a toner image adhered on the image bearing member to a recording material at a transfer region, and a guide member for restricting a conveying path of conveying the recording material to the transfer region.
2. Description of the Related Art
The electrophotographic type image forming apparatus electrically charges an image bearing drum in a uniform manner and then exposes it to form an electrostatic latent image, forms a toner image by development, transfers the toner image to a recording material, thermally fixes the toner image by fixed means, and then outputs the image-fixed recording material as an image-formed article (copy, print).
In these days, there have been used many transfer materials contacting the image bearing drum, as transfer materials for transferring the toner image from the image bearing drum to the recording material. There is an advantage that the transfer member contacting the image bearing drum reduces a bias voltage which is applied to the transfer member during the transfer.
A description will now be made of a conventional image forming apparatus.
FIG. 6 shows a conveying path of a transfer material Q between a guide member 3 and a transfer region in which a toner image on an image bearing drum 1 is transferred on to the recording material Q by a transfer member 5, in a conventional image forming apparatus.
As shown in FIG. 6, there are around the image bearing drum 1 an electrically charging roller 8 and a development roller 9. The recording material Q is nipped at a region (first contact region) where the transfer member 5 and the image bearing drum 1 contact each other. Applying a bias to the transfer member 5 from an electric source 17 causes the toner image to be transferred on to the recording material Q. The transfer member 5 is pressed to the image bearing drum 1 by a transfer member urging member 6. At a location upstream of the first contact region in a recording material-conveying direction, registration roller pair 2 are arranged, and a guide member 3 is arranged between the registration roller pair 2 and the transfer region.
The recording material Q contacts the guide member 3 at a second contact region, and then enters into the first contact region; accordingly, a positional relationship of the guide member 3 and the transfer region leads to definition of an angle of the transfer material Q entering into the transfer member 5.
When the recording material Q is conveyed to the first contact region along the transfer member 5, the toner image on the image bearing drum 1 is subjected to unfavorable transfer to thereby provide the image dispersion, before the recording material 5 enters into the first contact region under the influence of an electric field formed between the bias-applied transfer member 5 and the image bearing drum 1. In order to prevent the toner image-dispersion immediately before the first contact region (so-called pre-transfer phenomenon), it is so constructed that a tip of the guide member 3 is arranged on a side of the image bearing drum 1 with respect to a tangent line R of the image bearing drum 1 at a recording material conveying direction-wise center of the first contact region, and then the transfer material Q enters into the first contact region along the image bearing drum 1.
On the other hand, a surface of the image bearing drum 1 gets more scarred or scratched as the number of prints increases. When the image bearing drum 1 gets scarred or scratched over a predetermined amount, uneven electrical charge and so on causes an abnormal image to be formed on the print. Usually the life of the image bearing drum 1 is set to the printing-possible number of sheets in which the electrical charge-abnormal does not occur, and hence a long life image bearing drum which is capable of carrying out printing much more is desired in order to reduce the running cost. Therefore, an image forming apparatus using the transfer member 5 may be so constructed that the pressing force of the transfer member 5 pressing the image bearing drum 1 is reduced in order to prevent the image bearing drum 1 from getting scarred and scratched due to the transfer member 5 pressing the photosensitive drum 1 (Japanese Patent Application Laid-open No. 9-127804)
Moreover, when the pressing force is too weak, the photosensitive drum 1 easily has a shock due to a change of a sheet stance during the sheet conveying, thereby possibly providing poor images. Therefore, it is required to increase the pressing force of the transfer member 5 pressing the photosensitive drum 1 over a predetermined value in order to stably convey a paper.
However, in the above-mentioned image forming apparatus in which the pressing force of the transfer member 5 pressing the image bearing drum 1 is reduced, when using a high rigid heavy paper as the recording paper Q, the pressing force of the recording material going down the transfer member is superior to the pressing force of the transfer member 5 pressing the image bearing drum 1 due to the principle of leverage, thereby causing the transfer member 5 to separate from the image bearing drum 1, which provides the poor transfer. The poor transfer occurring by the transfer member 5 separating from the image bearing drum 1 causes the image appearance quality to be more damaged, compared with the poor image due to the pre-transfer phenomenon. Accordingly, when using a high rigid heavy paper as the recording material Q, it is more important to prevent the poor transfer occurring by the transfer member separating from the image bearing drum, compared with the poor image due to the pre-transfer phenomenon.
Here, there occurs a problem that it is difficult to prevent the transfer member from separating from the image bearing drum when using a high rigid recording material such as a heavy paper, and simultaneously reduce the occurrence of the pre-transfer phenomenon when using a low rigid recording material such as a thin paper.